Heat Flux Converter
Same value in every unit
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Power per unit area, and why that trips people up
Heat flux is a rate divided by an area: how quickly energy crosses one square metre of surface. That division is where the confusion starts. A 2 kW heater has a fixed power; the heat flux leaving its front face depends entirely on how large that face is. Spread the same 2 kW over twice the area and the flux halves, even though nothing about the heater has changed.
Moving between the two is arithmetic, not unit conversion. Multiply flux by the area it acts on to get total power; divide total power by area to get flux. Sunlight at 1000 W/m² falling square-on to 20 m² of roof is 20 kW arriving, of which a 20 per cent efficient array turns about 4 kW into electricity.
Where the 1000 W/m² solar figure comes from
Above the atmosphere a surface facing the Sun receives about 1361 W/m² — the solar constant, quoted at the mean Earth–Sun distance and swinging roughly ±3.4 per cent over the year as that distance changes. After scattering and absorption by air, water vapour and dust, roughly 1000 W/m² reaches the ground on a clear day with the Sun high. That rounded value is the standard test condition panels are rated at (1000 W/m², 25 °C cell temperature, AM1.5 spectrum), so nameplate watts describe a bright, cool, best-case moment rather than a typical one.
Rates, totals and near-miss units
W/m² is instantaneous. Energy accumulated over time is a different quantity: 1 kW/m² sustained for an hour is 1 kWh/m², which is how solar resource data is normally published. Two look-alikes are worth watching. BTU per hour-square foot and kilocalorie per hour-square metre both contain an hour, but they are still rates — the hour belongs to the power term, not to an accumulation. And W/(m²·K) is a U-value: flux per kelvin of temperature difference, so it will not convert against anything on this page. Calorie units carry a small ambiguity of their own: the thermochemical calorie is 4.184 J and the International Table calorie 4.1868 J, about 0.07 per cent apart.
Reference points
- Solar constant, top of atmosphere: about 1361 W/m²
- Clear sky at ground level, Sun high: about 1000 W/m², which is 0.1 W/cm²
- That same 1000 W/m² in imperial terms: about 317 BTU/h·ft²
- Resting adult, averaged over skin area: about 58 W/m² (the met unit used in thermal comfort work)
- Mean geothermal heat flow out of the Earth's surface: roughly 0.09 W/m², i.e. about 90 mW/m²
- Water boiling on a hot surface at atmospheric pressure, near the critical heat flux: of order 1 MW/m², i.e. about 100 W/cm²
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Frequently Asked Questions
It measures power crossing a surface, per square metre of that surface. One W/m² is one joule per second passing through each square metre. The figure says nothing about how large the surface is, so two very different objects can share the same heat flux while transferring wildly different total amounts of energy.
Power is the total rate of energy transfer; heat flux is that rate spread over an area. Multiply flux by area to get power, and divide power by area to get flux. A 1500 W panel heater and sunlight at 1000 W/m² are not comparable numbers until you fix an area for the heater: spread over 1 m² its output is 1500 W/m² and beats the sunlight, spread over 3 m² it is 500 W/m² and does not.
About 1000 W/m² at ground level on a clear day with the Sun near overhead. Above the atmosphere it is roughly 1361 W/m², the solar constant, and the gap is what air, water vapour and dust scatter or absorb. Low sun angles, haze and cloud cut the ground figure sharply, so 1000 W/m² is a bright-day benchmark rather than an average — brief cloud-edge reflections and high-altitude sites can push past it.
Because standard test conditions define it that way: 1000 W/m² irradiance, 25 °C cell temperature and an AM1.5 spectrum. It is a laboratory benchmark chosen so panels from different makers can be compared on the same basis. Real installations rarely meet all three conditions at once, since bright days usually push cell temperature well above 25 °C, which is why measured output sits below nameplate most of the time.
Multiply by 3.154590745. So 100 BTU/h·ft² is about 315 W/m², and clear-sky sunlight at 1000 W/m² works out at about 317 BTU/h·ft². Both units are rates already: the hour belongs to the BTU-per-hour power term, so you are converting a flux, not an amount of energy.
No. W/(m²·K) is a U-value or heat transfer coefficient — heat flux per kelvin of temperature difference across the surface — so it is a different physical quantity. Multiply it by the actual temperature difference to get a heat flux you can convert here. A wall with a U-value of 0.3 W/(m²·K) and 20 K across it passes 6 W/m².